The present invention relates to fiber optic transmission and more particularly to wavelength division multiplex transmission using dispersion shifted fiber as the line fiber.
The index profile of optical fibers is generally characterized as a function of the shape of the graph of the function that associates the radius of the fiber and the refractive index. It is conventional to plot on the abscissa axis the distance r to the center of the fiber and on the ordinate axis the difference between the refractive index and the refractive index of the cladding of the fiber. The expressions xe2x80x9cstep profilexe2x80x9d, xe2x80x9ctrapezium profilexe2x80x9d and xe2x80x9ctriangle profilexe2x80x9d are used to refer to graphs which respectively have step, trapezium and triangle shapes. These curves are generally representative of the theoretical profile or set point profile of the fiber and fiber fabrication constraints can yield a substantially different profile.
To use a fiber in a transmission system, and in particular in a wavelength division multiplex transmission system, it is beneficial for the fiber to have a large effective surface area in the range of wavelengths of the multiplex. A large effective surface area limits the power density in the fiber, at total constant power, and limits or prevents undesirable non-linear effects.
For high bit rate systems, it is also beneficial for the fiber to assure monomode propagation of the channels of the multiplex. ITU-T Recommendation G.650 defines the in-cable cut-off wavelength. The theoretical cut-off wavelength of the fiber is generally several hundred nanometers greater than the in-cable cut-off wavelength. It appears that propagation in an optical fiber can be monomode, even if the theoretical cut-off wavelength is greater than the wavelength of the signals used: beyond a distance of a few meters or tens of meters, which is small in comparison with the propagation distances in fiber optic transmission systems, the secondary modes disappear because of excessive attenuation. Propagation in the transmission system is then monomode.
It is also important for the fiber to have as small a sensitivity as possible to bends and microbends. The sensitivity to bends is evaluated, as explained in ITU-T Recommendation G.650, by measuring the attenuation caused by winding 100 turns of a fiber around a 30 mm diameter spool. The sensitivity to microbends is measured in a manner that is well known in the art; as described hereinafter, it can be measured relative to a fiber such as the ASMF 200 fiber manufactured by the assignees of the applicants.
In new high bit rate wavelength division multiplex transmission networks it is advantageous to limit the chromatic dispersion slope in the range of wavelengths of the multiplex; the objective is to minimize distortion between channels of the multiplex during transmission.
Dispersion shifted fibers (DSF) are now commercially available. Their chromatic dispersion is substantially zero at the transmission wavelength at which they are used, the chromatic dispersion generally being different from the wavelength of 1.3 xcexcm at which the dispersion of the silica is substantially zero. In other words, the non-zero chromatic dispersion of the silica is compensated by an increase in the index difference xcex94n between the fiber core and the optical cladding. This index difference shifts the wavelength for zero chromatic dispersion; it is obtained by introducing dopants into the preform during its fabrication, for example by an MCVD process well known in the art and not described in detail here. Non-zero dispersion shifted fibers (NZ-DSF) are dispersion shifted fibers which have non-zero chromatic dispersion at the wavelengths at which they are used. The non-zero chromatic dispersion limits non-linear effects in the fiber and in particular four-wave mixing between the channels of the multiplex.
Document EP-0 883 002 relates, with reference to its FIG. 3C, to in-cable monomode DSF fibers, having a step and ring profile and an average chromatic dispersion slope of 0.043 ps/nm2xc2x7km. However, those fibers have negative chromatic dispersion of about 1550 nm.
The problem with DSF, as explained in document EP-A-0 859 247, is that the chromatic dispersion slope generally increases as the effective surface area increases.
EP-A-0 859 247 describes ring profile DSF and explains that for such fibers there is a range in which the effective surface area and the chromatic dispersion slope vary in different directions. The fibers referred to by way of example have a negative chromatic dispersion in the range xe2x88x924.5 ps/(nmxc2x7km) to xe2x88x921.0 ps/nmxc2x7km. They have a cut-off wavelength greater than 1500 nm for a fiber length of 2 m. The above document specifies that the high cut-off wavelength is not a problem because the cut-off wavelength decreases with the propagation distance and monomode propagation is assured for transmission distances in the order of 1000 km.
The invention proposes an optical fiber that can be used in a cable and which represents an advantageous compromise between the effective surface area and the chromatic dispersion slope, in particular by virtue of the chosen cut-off wavelength, and which is additionally easy to fabricate.
To be more precise, the invention consists in an in-cable monomode optical fiber comprising an optical core surrounded by optical cladding, said optical core presenting an index profile constituted by a stepped central part, surrounded by an intermediate zone of refractive index less than that of said central part, itself surrounded by an annular zone having refractive index less than that of said central part and greater than that of said intermediate zone, said fiber having, for a wavelength of 1550 nm, a chromatic dispersion slope in the range 0 to 0.1 ps/nm2xc2x7km, said fiber being characterized in that it further presents the following features for a wavelength of 1550 nm:
an effective surface area greater than or equal to 60 xcexcm2,
a chromatic dispersion in the range 3 ps/(nmxc2x7km) to 14 ps/(nmxc2x7km), and
a ratio between the effective surface area and the chromatic dispersion slope greater than 900 xcexcm2xc2x7nm2xc2x7km/ps.
The fiber of the invention has, simultaneously, a strong effective surface area, positive chromatic dispersion of about 1550 nm, and a chromatic dispersion slope which remains small. It has the advantage of satisfying requirements concerning bending losses and sensitivity to microbends, while being easy to fabricate.
The fiber of the invention preferably has chromatic dispersion at 1550 nm in the range 5 ps/nmxc2x7km to 11 ps/nmxc2x7km and/or a chromatic dispersion slope less than 0.07 ps/nm2xc2x7km.
The fiber of the invention preferably has a ratio between the effective surface area and the chromatic dispersion slope greater than or equal to 1000 xcexcm2xc2x7nm2xc2x7km/ps. This ratio is preferably less than or equal to 5000 xcexcm2xc2x7nm2xc2x7km/ps or even 2500 xcexcm2xc2x7nm2xc2x7km/ps.
The fiber of the invention preferably has a chromatic dispersion cancellation wavelength xcex0 less than or equal to 1480 nm.
In one embodiment of the invention the fiber has an effective surface area greater than or equal to 70 xcexcm2.
The fiber of the invention has bending losses at 1550 nm less than or equal to 0.05 dB and preferably less than or equal to 0.005 dB for 100 turns of fiber wound with a radius of 30 mm. It can also have a sensitivity to microbends less than 1.2 and preferably less than 0.8.
The fiber preferably has a theoretical cut-off wavelength greater than 1550 nm and an in-cable cut-off wavelength less than 1300 nm.
In one embodiment of the invention the fiber has an attenuation at 1550 nm less than or equal to 0.23 dB/km and a polarization modal dispersion less than or equal to 0.1 psxc2x7kmxe2x88x920.5.
In advantageous manner, the difference between the index of the central part of the fiber and the index of the optical cladding can lie in the range 5xc3x9710xe2x88x923 to 9xc3x9710xe2x88x923. In this case, the ratio between the radius of the central part and the outside radius of the ring is advantageously in the range 0.23 to 0.45.
Also advantageously, the difference between the index of the intermediate zone and the index of the optical cladding can lie in the range xe2x88x924xc3x9710xe2x88x923 to 1xc3x9710xe2x88x923 and is preferably in the range xe2x88x923xc3x9710xe2x88x923 to 5xc3x9710xe2x88x924. In this case, the ratio between the outside radius of the intermediate zone and the outside radius of the ring can lie in the range 0.45 to 0.75 and is preferably in the range 0.48 to 0.7.
In an advantageous embodiment, the difference between the index of the ring and the index of the optical cladding can lie in the range 5xc3x9710xe2x88x924 to 5xc3x9710xe2x88x923. In this case, the outside radius of the ring is advantageously in the range 7 xcexcm to 13 xcexcm.
In addition, the parameters of the fiber of the invention can advantageously be chosen in such a manner as to satisfy one or more of the following relationships, in which r is the radius and xcex94n(r) is the index difference between the index at radius r and the index of the optical cladding:       S    1    =      2    ·                  ∫        0                  r          1                    ⁢              Δ        ⁢                  xe2x80x83                ⁢                              n            ⁢                          (              r              )                                ·          r          ·                      xe2x80x83                    ⁢                      ⅆ            r                              
(where r1 is the radius of the central part), in the range 45xc3x9710xe2x88x923 xcexcm2 to 110xc3x9710xe2x88x923 xcexcm2 and is preferably in the range 50xc3x9710xe2x88x923 xcexcm2 to 110xc3x9710xe2x88x923 xcexcm2.       S    12    =      2    ·                  ∫        0                  r          2                    ⁢              Δ        ⁢                  xe2x80x83                ⁢                              n            ⁢                          (              r              )                                ·          r          ·                      xe2x80x83                    ⁢                      ⅆ            r                              
(where r2 is the outside radius of the intermediate zone), greater than 25xc3x9710xe2x88x923 xcexcm2 and is preferably in the range 30xc3x9710xe2x88x923 xcexcm to 100xc3x9710xe2x88x923 xcexcm2.       S    23    =      2    ·                  ∫                  r          1                          r          3                    ⁢              Δ        ⁢                  xe2x80x83                ⁢                              n            ⁢                          (              r              )                                ·          r          ·                      xe2x80x83                    ⁢                      ⅆ            r                              
(where r3 is the outside radius of the annular zone), in the range 30xc3x9710xe2x88x923 xcexcm2 to 150xc3x9710xe2x88x923 xcexcm2 and is preferably in the range 55xc3x9710xe2x88x923 xcexcm2 to 140xc3x9710xe2x88x923 xcexcm2.       S    123    =      2    ·                  ∫        0                  r          3                    ⁢              Δ        ⁢                  xe2x80x83                ⁢                              n            ⁢                          (              r              )                                ·          r          ·                      xe2x80x83                    ⁢                      ⅆ            r                              
less than 220xc3x9710xe2x88x923 xcexcm2 and is preferably in the range 130xc3x9710xe2x88x923 xcexcm to 205xc3x9710xe2x88x923 xcexcm2.
The invention also proposes a wavelength division multiplex fiber optic transmission system comprising a fiber as defined hereinabove as line fiber. It can then further comprise dispersion compensating fiber.